Power conversion devices for electric vehicles have hitherto required a high-voltage battery as a power supply.
FIG. 1 is a circuit diagram showing a conventional inverter for an electric vehicle. Inverter 502 converts the DC power from battery 503 to three-phase AC power by typically using semiconductor switches 601-606 as shown in FIG. 1 to drive AC motor 501. In this inverter 502, the maximum voltage that can be applied to AC motor 501 as interphase voltage is equal to the battery voltage due to the connections of semiconductor switches 601-606. A relatively inexpensive 200 V motor is typically used as AC motor 501. A DC voltage higher than 282 V (.apprxeq.200.times.2.sup.1/2), which is the amplitude of AC 200 V, is therefore required as the minimum input voltage of inverter 502 in order to obtain three-phase 200 V AC power as an AC output from inverter 502. A battery having a voltage of approximately 300 V is thus necessary to drive the 200-V motor.
To produce high voltage in a battery, however, 110 to 190 batteries each having cells of several volts each must be connected in series. A additionally, to charge a large number of batteries connected in series, a high-voltage direct-current voltage higher than that of the batteries must be produced by means of a charger. When charging a battery in which a large number of cells are connected in series, moreover, the voltages of each of the cells cannot be completely equalized due to the individual differences between the characteristics of each of the cells, and thereby causing variations in the charging conditions of each cell. The number of serial connections in the battery should therefore be reduced to a minimum to lower the charge voltage. Lowering the battery voltage however results in insufficient direct-current voltage for the AC motor.